Spread spectrum modulation techniques increase the bandwidth of a signal by using a code sequence which is known by both the transmitter and the receiver. In direct sequence spread spectrum (DSSS) systems, the information signal is directly modulated by the code sequence, which is referred to as a spreading code or pseudorandom (PN) code. Code Division Multiple Access (CDMA) is a direct sequence spread spectrum technology for sharing resources in a cellular telephone system. Within a given cell, multiple users must share the frequency spectrum. One method of sharing the spectrum is frequency division, in which the frequency spectrum is divided into multiple frequency channels and each mobile station is allocated a frequency channel to communicate with the base station of the cell. This method is known as frequency division multiple access (FDMA). In another method known as time-division multiple access (TDMA), multiple mobile units share a frequency channel but use it at different times.
Unlike FDMA and TDMA, CDMA allows every mobile unit to communicate with the base station using same frequency channel at the same time. As mentioned above, in CDMA, the baseband information signal is modulated using a code sequence, known as a spreading code. The spreading code is made up of a plurality of elements known as chips. The chip rate (i.e., the number of chips transferred per second) is typically higher than the symbol rate (i.e., the number of data symbols, made up of one or more data bits, transferred per second) of the baseband information signal. The result is that the modulated signal is spread over a much wider frequency spectrum than the baseband information signal.
In a CDMA cellular system, each mobile unit uses a different set of spreading codes to communicate with the base station. The spreading codes are selected to have low cross-correlation with each other. That is, the spreading codes are designed for maximum separation from each other. The base station can identify each transmitting mobile unit based on the spreading code used in the transmission. Similarly, the mobile station may identify and communicate with a base station based on a spreading code for that base station.
However, many different wireless standards use CDMA technology (e.g., IS-95, CDMA200, WCDMA, TD-SCDMA, etc.). These standards are incompatible with each other, as they use different modulation techniques, chip rates, PN codes and protocols.
CDMA communication systems offer many advantages over narrowband systems. For example, multipath signals can cause interference in a narrowband system. Multipath may be created by reflection of signals from objects in the environment, such as trees, buildings, and cars. As illustrated in FIG. 1, a signal transmitted from base station 100 to mobile station 106 has three diverse signal paths. A signal on signal path 108 is reflected from building 102 before reception by mobile station 106. A signal on signal path 110 is reflected from building 104 before reception by mobile station 106. There is no reflection in signal path 112, as the path is directly from base station 100 to mobile station 106. Because signal paths 108, 110, and 112 have different lengths between the base station and the mobile station, each signal may be received at a different time. Unlike narrowband systems, in which such multipath signals may pose a problem known as intersymbol interference, separate multipath signals may be distinguished and separately received in a CDMA system. In addition, signal components received on diverse paths may be aligned in time and combined to produce a stronger signal.
A RAKE receiver is typically employed to process multipath signal components. A RAKE receiver typically includes multiple “fingers,” each of which receives and despreads one of the multipath signal components. A finger compensates for delay via associated synchronization (for example delayed lock loop) and correlates a signal received on one of the diverse paths with a spreading code to demodulate the signal and recover the original baseband signal.
A typical RAKE architecture is shown in FIG. 2. In FIG. 2, RAKE fingers 202a-202n receive signal components from an input buffer 200. The delay between paths in the channel is represented by the tap position in the input buffer of each RAKE finger 202. The RAKE fingers 202 despread multipath signals which are subsequently combined to produce a stronger signal. Such RAKE receivers are typically designed to comply with a particular wireless standard. Thus, custom hardware logic is typically provided in the RAKE receiver for using specific spreading codes, chip rates, modulation techniques, and protocols required by a particular wireless standard. As a result, these receivers lack flexibility and operate in accordance with a particular wireless standard. As a result, the mobile station (i.e., the user's cellular telephone) is restricted to operation with the standard and may need to be replaced if the standard is altered or becomes obsolete. From the viewpoint of the mobile station manufacturer, different chip designs are required for different standards. Similar problems exist with RAKE receivers used in base stations.
Accordingly, there is a need for improved methods and apparatus for spread spectrum signal processing in wireless communication systems.